Optical switches are needed for routing signals in optical fiber communication systems. Two basic operating principles are used in known devices. These principles are free space optics and planar waveguides.
Free space switches use collimators to generate optical beams traveling in free space. These optical beams can be routed by moveable mirrors and other similar devices to receiving collimators positioned on the desired output fibers. It is known that small optical beams will diverge as they travel, due to diffraction. This divergence causes large losses in devices that have practical sizes. In addition, practical limits on the flatness of the moveable mirrors cause additional divergence and further losses. Further still, collimators are large, expensive and very difficult to align, all factors that cause free-space switches to be expensive to manufacture.
Planar optical waveguides have been used to eliminate the beam divergence inherent in the free space devices described above. Planar optical waveguides can also eliminate the need for input and output collimators, resulting in a more compact structure with lower manufacturing costs. Waveguides of various known configurations are formed on the surface of a substrate. Various switching mechanisms are used to route the signals at the intersections of these surface waveguides. The 2-dimensional nature of these devices generally requires an air gap at these intersections so that a switching mechanism can be inserted. Moveable mirrors and bubbles in optical index matching coupling fluid have been used to create this switching mechanism.
Known devices have large losses at these intersection due to the presence of the air gap. An N×N switch will have 2N such intersections. These losses become unacceptable as N becomes large. In addition, planar waveguides do not have light beam profiles that match those of an optical fiber. This causes substantial coupling losses at the input and output stages where fiber coupling is to occur.
Previous patent applications by this inventor (U.S. application Ser. No. 09/905,736 entitled “Optical Switch with Moveable Holographic Optical Element” and Ser. No. 09/905,769 entitled “Integrated Transparent Substrate and Diffractive Optical Element,” each expressly incorporated herein by reference) show a switch that combines the advantages of free space and waveguide devices. The approaches shown are generally illustrated in FIGS. 1 and 2. These applications show a switch that is based on routing of optical signals via total internal reflection (TIR) in a transparent substrate. The configurations reduce beam divergence because of the higher index of refraction in the substrate as compared to free-space. These configurations also minimize alignment and positioning problems since all of the components are rigidly and precisely located by the substrate. The devices eliminate the air gaps that are required in known planar waveguide based switches, since total internal reflection is used to route the signals. Total internal reflection is known to have very little loss, and this mechanism eliminates the loss problem inherent in such waveguide switches.
In operation, a diffraction grating 100 is disposed adjacent an optical substrate 102 having an incident light beam 104 traveling within the substrate 102 under total internal reflection (TIR), which occurs above a critical incidence angle. The diffraction grating 100 is moveable relative to the substrate 102 to selectively introduce the diffraction grating 100 into the evanescent field generated at a upper surface 106 of the substrate 102 where TIR occurs. The diffraction grating 100 illustrated in FIGS. 1 and 2 is formed from parallel strips 108. FIG. 1 shows the diffraction grating 100 in a first, switching position, where the input signal 104 is switched into an output beam 110. FIG. 2 shows a second, non-switching position, where the diffraction grating 100 does not affect the input wave 104, which continues to propagate via TIR as an output beam 112. The deflection of the beam 104 into light beam 110 represents beam switching, while the reflection into light beam 112 represents un-affected propagation.
The diffraction grating 100 is typically designed to have a single diffraction mode, the −1 diffraction mode, which results in maximum power being directed in a desired direction, i.e., light beam 110 or 112. This minimizes loss in switching position, as compared to the virtually loss-free non-switching position. The thickness of the grating strips 108 may be adjusted so that the light reflected from the diffraction grating 100 is in phase with the light reflected at the surface 106 in the desired direction. This results in constructive interference and the diffraction grating 100 can have an overall efficiency of approximately 90%.
In spite of these advantages, generally devices like those of FIGS. 1 and 2 may still require collimators to minimize beam spreading. In addition, the relatively long path between grating and output fibers may introduce wavelength dependent loss (WDL). This is undesirable in telecommunications systems and should be minimized. The WDL is due to grating dispersion, where different wavelengths propagate in slightly different directions. This effect could limit the practical N value for an N×N switch using these approaches.